The US has warned its citizens that Europe is facing a "heightened risk of terror attacks" at Christmas markets and other seasonal holiday events.

The US State Department said it had "credible information" that Isil and al-Qa'ida were planning attacks and focusing on the "upcoming holiday season".

It specifically warned American travellers to exercise caution at "holiday festivals, events and outdoor markets".

The travel warning was issued as the US military said it expected Isil to resort to more traditional terrorist attacks after it loses its twin-city strongholds of Mosul and Raqqa, both of which are under pressure.

"We don't think they are going to become an organisation that no longer presents a danger once Raqqa and Mosul are taken away," said Col John Dorrian, the spokesman for the US-led coalition against Isil.

"What they are going to do is devolve into the type of terror organisation that we've known they were all along and continue to try to do external operations and try to motivate lone wolf attackers."

The State Department warning was released a week after the one-year anniversary of the 2015 Paris attacks, which killed 130 people across the French capital. It does not mention any specific countries as being particularly at risk but notes that the last year has seen attacks in Belgium, France, Germany, and Turkey.

"The Department of State alerts US citizens to the heightened risk of terrorist attacks throughout Europe, particularly during the holiday season. US citizens should exercise caution at holiday festivals, events, and outdoor markets," the warning said.

"Credible information indicates the Islamic State of Iraq and the Levant (Isil or Daesh), al-Qa'ida, and their affiliates continue to plan terrorist attacks in Europe, with a focus on the upcoming holiday season and associated events.

"US citizens should also be alert to the possibility that extremist sympathisers or self-radicalised extremists may conduct attacks during this period with little or no warning. Terrorists may employ a wide variety of tactics, using both conventional and non-conventional weapons and targeting both official and private interests."

The warning followed the arrest of seven men, revealed on Monday. French anti-terrorism police arrested the men in Strasbourg and Marseille. They had links to Syria and had been plotting an attack on France for several months, the interior minister said.

A series of arrests in June and on Sunday put an end to the plot and "allowed us to thwart a terrorist act that had been envisaged for a long time on our soil", Bernard Cazeneuve told reporters in Paris.

France is still under a state of emergency imposed after deadly Isil attacks on Paris last year.

The suspects are believed to have initially wanted to target the Euro 2016 football tournament earlier this year but later focused on another target, possibly in Paris or Marseille, according to a security official.

The June arrests involved people behind financing the alleged attack plot, while the Sunday arrests targeted the operational team - and both groups were under orders from unidentified commanders in Syria, the official said. Weapons were also seized in Sunday's arrests, the official said. Mr Cazeneuve said investigators were studying whether the thwarted attack was part of a larger plot to attack multiple sites simultaneously.

Five of the suspects are French, one is Moroccan and the other Afghan, and they are aged between 29 and 37, according to the security official.

Mr Cazeneuve said six of them had not been known to intelligence services.

The Moroccan had apparently been living in Portugal.

Portuguese police said that they had flagged a 26-year-old Moroccan residing in Aveiro in northern Portugal to other European authorities, warning that he was part of a terrorist group.

In a statement, the police said they had been watching him since 2015, and he was arrested by French police over the weekend.